Electron microscopy was performed on several minute tumors of the type called chemodectomas, all from the lung of a single patient. The cells had a whorling pattern with extensive interdigitating cytoplasmic processes joined by desmosomes. Except for tangles of cytoplasmic fibrils, the tumor cells had few distinctive organelles. They had no endocrine-like granules and were not associated with nerves or basement membranes. The tumors had little resemblance to paragangliomas, but displayed a puzzling similarity to meningiomas. Our observations permit no definite conclusions as to the histogenesis of these lung tumors. Viewed in the light of recent physiologic studies, they cast doubt on the presence of special chemoreceptive paraganglia in the lung.